Debugging 554 Responses

Since my migration to AWS and to James 3.4.x, I'm getting several '554' 
bounce responses from certain recipient servers.  I know this is a long 
shot on this question.  But any advice will be greatly appreciated.

I've sent mail to mail-tester.com which has been incredibly useful in 
resolving configuration issues in the past.  My mail is receiving a 10 
out of 10 perfect score.  This means rDNS, SPF, DKIM, SpamAssassin, 
blacklists, etc all are happy with my email. I even went so far as 
requesting 2 additional IP addresses from AWS.  I reconfigured JAMES 
several times to use the different IP addresses (and their equivalent 
rDNS names) in the transport mailet.  Each time, I rechecked with 
mail-tester.com and still got 10/10 with each different IP address.  
Yet, certain receiving domains are still bouncing with a 554.  It's hard 
to believe that three different IP addresses I received from AWS are all 
blacklisted on some DNSBL site.  But I'm definitely not the one causing 
a blacklisting.  The first emails I sent after changing to the 2nd and 
3rd IP addresses bounced with the 554.

I completely understand that a recipient server can bounce an email for 
any random reason.  It's just frustrating that they don't include any 
reason at all other than generic 554 which pretty much means nothing 
more than "you ain't gettin' in"

One instance of this problem is from a local school district domain.  
I'm trying to locate the IT support person to try get more info on the 
bounce.  But in the meantime, has anybody else had experiences like this 
and might have an idea of what I might still be doing wrong that is so 
offensive to these receiving server(s)?

Thanks.

Jerry